The author assumes that the question can be answered by applying principles of anatomy, physics and evolutionary biology. First reason he suggests is that cats are small and they have a favourable mass-surface area ratio. Next, he suggests that cats have soft tissues that absorb the shock and they land on all four limbs, dividing the shock. This is because of their body shape and its centre of gravity. He says that felines have this kind of body structure (that canines lack) because years of falling from trees etc. has evolved their body in this way.
